1. Field of the Invention
This invention relates to the processing of cold-rolled stainless-steel sheet or strip, and in particular, to a method of annealing and then descaling that product.
2. Description of the Prior Art
In the manufacture of flat-rolled stainless-steel sheet and strip products, it is necessary to anneal or soften the material, subsequent to the cold-rolling operation. One very common annealing practice involves heating the steel in an oxidizing furnace atmosphere to a temperature at which oxide surface scale readily forms. Such heating, to a temperature typically in the range of 1450 degrees Fahrenheit to 2150 degrees Fahrenheit (788 degrees Centigrade to 11177 degrees Centigrade), depending upon the chemical composition of the steel being processed, is commonly done in a refractory-lined, gas-fired furnace containing an oxidizing atmosphere, and such treatment typically results in the formation on the stainless steel of an oxide scale having a thickness in the range of 4000 Angstroms to several microns. Such a scale must ultimately be removed before the product is considered ready for sale. To accomplish this, according to the prior art, any of several different descaling processes may be used, alone or in combination, and among these known processes there are (1) shot blasting following by acid pickling, (2) conditioning the scale by immersing the product in molten salt or subjecting it to an electrolytic treatment, followed by acid pickling, and (3) straight acid pickling. The acids commonly used in the pickling operation include sulfuric, nitric, and nitric-hydrofluoric combinations. The use of such acids is disadvantageous and costly, not only because the acids themselves are not cheap but also because they are hazardous materials which necessitate the use of special handling techniques before, during, and after their use; moreover, because of environmental regulations, the disposal of waste pickle liquor presents costly problems.
The prior art also contains U.S. Pat. No. 4,363,709, which discloses the removal of oxide scale from the surface of a metallic body of stainless steel, with the use of a high-current-density electrolytic descaling process in a bath consisting of an aqueous solution containing about 15 to 25 weight percent of sodium sulfate, maintained at a temperature of at least 150 degrees Fahrenheit (65.6 degrees Centigrade). According to the above-mentioned patent, the stainless steel is subjected as the anode to the action of a direct electric current for at least 10 seconds at a current density of at least 3 amperes per square inch (46.5 Amps/dm.sup.2).
The prior art contains, moreover, a group of patents which are concerned with the problem of providing heat to metal strip or sheet material by means of electrical induction. This prior art includes, for example, the following U.S. Pat. Nos.: 4,054,770; 4,585,916; 3,444,346; 2,902,572; and 4,678,883. These patents may be considered as relating to such an electromagnetic induction process called "transverse flux induction heating".
There have been factors which would lead persons skilled in the art to seek to substitute induction heating for the conventional gas-fired heating in a refractory-lined furnace with an oxidizing atmosphere. It has been apparent to those skilled in the art that if this can be done, the heating should be more rapid and efficient, possibly causing the development of the desired annealing with the development of an oxide scale of lesser thickness because of a decreased time at high temperature. At the same time, however, those skilled in the art have also appreciated that there are considerable practical difficulties which have tended to make this apparently desirable substitution difficult or impossible.
For example, there has been the problem that unless particular measures are taken to decrease the amount of magnetic flux applied in the vicinity of the edges of the strip or sheet, uneven heating develops, with the unwanted result that when an attempt is made to commercially process stainless-steel strip at a satisfactory throughput rate, intolerable buckling or cobbling of the strip being processed is encountered.
Moreover, the known gas-fired processes are capable of handling stainless-steel strip in a variety of strip widths without it being necessary to make any other important adjustments in the process parameters. With the induction heating, a change from one known commercial width to another, and a change in the composition in the stainless steel being processed, requires further experimentation to determine another satisfactory set of operating conditions.
The state of the prior art has been such that those skilled in the art have not been aware that it is indeed feasible to practice a method for the processing of cold-rolled stainless steel sheet or strip, providing the annealing and subsequent descaling thereof by a combination of induction heating and subsequent modified electrolytic treatment in sodium sulfate solution, such that when properly performed, the requirements for subsequent pickling can be entirely avoided.